Lack of relationship between muscle sympathetic nerve activity and skeletal muscle vasodilation in response to insulin infusion

Role of the arterial baroreflex in the sympathetic response to hyperinsulinemia in adult humans

Muscle sympathetic nerve activity (MSNA) increases during hyperinsulinemia, primarily attributed to central nervous system effects. Whether peripheral vasodilation induced by insulin further contributes to increased MSNA via arterial baroreflex-mediated mechanisms requires further investigation. Accordingly, we examined baroreflex modulation of the MSNA response to hyperinsulinemia. We hypothesized that rescuing peripheral resistance with coinfusion of the vasoconstrictor phenylephrine would attenuate the MSNA response to hyperinsulinemia. We further hypothesized that the insulin-mediated increase in MSNA would be recapitulated with another vasodilator (sodium nitroprusside, SNP). In 33 young healthy adults (28 M/5F), MSNA (microneurography) and arterial blood pressure (BP, Finometer/brachial catheter) were measured, and total peripheral resistance (TPR, ModelFlow) and baroreflex sensitivity were calculated at rest and during intravenous infusion of insulin (n = 20) or SNP (n = 13). A subset of participants receiving insulin (n = 7) was coinfused with phenylephrine. Insulin infusion decreased TPR (P = 0.01) and increased MSNA (P < 0.01), with no effect on arterial baroreflex sensitivity or BP (P > 0.05). Coinfusion with phenylephrine returned TPR and MSNA to baseline, with no effect on arterial baroreflex sensitivity (P > 0.05). Similar to insulin, SNP decreased TPR (P < 0.02) and increased MSNA (P < 0.01), with no effect on arterial baroreflex sensitivity (P > 0.12). Acute hyperinsulinemia shifts the baroreflex stimulus-response curve to higher MSNA without changing sensitivity, likely due to insulin's peripheral vasodilatory effects. Results show that peripheral vasodilation induced by insulin contributes to increased MSNA during hyperinsulinemia.NEW & NOTEWORTHY We hypothesized that elevation in muscle sympathetic nervous system activity (MSNA) during hyperinsulinemia is mediated by its peripheral vasodilator effect on the arterial baroreflex. Using three separate protocols in humans, we observed increases in both MSNA and cardiac output during hyperinsulinemia, which we attributed to the baroreflex response to peripheral vasodilation induced by insulin. Results show that peripheral vasodilation induced by insulin contributes to increased MSNA during hyperinsulinemia.

Role of the Autonomic Nervous System in the Hemodynamic Response to Hyperinsulinemia-Implications for Obesity and Insulin Resistance

PURPOSE OF REVIEW

Herein, we summarize recent advances which provide new insights into the role of the autonomic nervous system in the control of blood flow and blood pressure during hyperinsulinemia. We also highlight remaining gaps in knowledge as it pertains to the translation of findings to relevant human chronic conditions such as obesity, insulin resistance, and type 2 diabetes.

RECENT FINDINGS

Our findings in insulin-sensitive adults show that increases in muscle sympathetic nerve activity with hyperinsulinemia do not result in greater sympathetically mediated vasoconstriction in the peripheral circulation. Both an attenuation of α-adrenergic-receptor vasoconstriction and augmented β-adrenergic vasodilation in the setting of high insulin likely explain these findings. In the absence of an increase in sympathetically mediated restraint of peripheral vasodilation during hyperinsulinemia, blood pressure is supported by increases in cardiac output in insulin-sensitive individuals. We highlight a dynamic interplay between central and peripheral mechanisms during hyperinsulinemia to increase sympathetic nervous system activity and maintain blood pressure in insulin-sensitive adults. Whether these results translate to the insulin-resistant condition and implications for long-term cardiovascular regulation warrants further exploration.

Effect of ingesting a meal and orthostasis on the regulation of splanchnic and systemic hemodynamics and the responsiveness of cardiovascular α1-adrenoceptors

Postprandial orthostasis activates mechanisms of cardiovascular homeostasis to maintain normal blood pressure (BP) and adequate blood flow to vital organs. The underlying mechanisms of cardiovascular homeostasis in postprandial orthostasis still require elucidation. Fourteen healthy volunteers were recruited to investigate the effect of an orthostatic challenge (60°-head-up-tilt for 20 min) on splanchnic and systemic hemodynamics before and after ingesting an 800-kcal composite meal. The splanchnic circulation was assessed by ultrasonography of the superior mesenteric and hepatic arteries and portal vein. Systemic hemodynamics were assessed noninvasively by continuous monitoring of BP, heart rate (HR), cardiac output (CO), and the pressor response to an intravenous infusion on increasing doses of phenylephrine, an α₁-adrenoceptor agonist. Neurohumoral regulation was assessed by spectral analysis of HR and BP, plasma catecholamine and aldosterone levels and plasma renin activity. Postprandial mesenteric hyperemia was associated with an increase in CO, a decrease in SVR and cardiac vagal tone, and reduction in baroreflex sensitivity with no change in sympathetic tone. Arterial α₁-adrenoceptor responsiveness was preserved and reduced in hepatic sinusoids. Postprandial orthostasis was associated with a shift of 500 mL of blood from mesenteric to systemic circulation with preserved sympathetic-mediated vasoconstriction. Meal ingestion provokes cardiovascular hyperdynamism, cardiac vagolysis, and resetting of the baroreflex without activation of the sympathetic nervous system. Meal ingestion also alters α₁-adrenoceptor responsiveness in the hepatic sinusoids and participates in the redistribution of blood volume from the mesenteric to the systemic circulation to maintain a normal BP during orthostasis.NEW & NOTEWORTHY A unique integrated investigation on the effect of meal on neurohumoral mechanisms and blood flow redistribution of the mesenteric circulation during orthostasis was investigated. Food ingestion results in cardiovascular hyperdynamism, reduction in cardiac vagal tone, and baroreflex sensitivity and causes a decrease in α₁-adrenoceptor responsiveness only in the venous intrahepatic sinusoids. About 500-mL blood shifts from the mesenteric to the systemic circulation during orthostasis. Accordingly, the orthostatic homeostatic mechanisms are better understood.

Out-of-Clinic Sympathetic Activity Is Increased in Patients With Masked Uncontrolled Hypertension

Masked uncontrolled hypertension (MUCH) is defined as controlled automated office blood pressure (BP; AOBP <135/85 mm Hg) in-clinic in patients receiving antihypertensive medication(s) but uncontrolled BP out-of-clinic by 24-hour ambulatory BP monitoring (ABPM; awake ≥135/85 mm Hg). We hypothesized that MUCH patients have greater out-of-clinic sympathetic activity compared with true controlled hypertensives. Patients being treated for hypertension were prospectively recruited after 3 or more consecutive clinic visits. All patients were evaluated by in-clinic automated office BP, plasma catecholamines, and spot-urine/plasma metanephrines. In addition, out-of-clinic 24-hour ABPM, 24-hour urinary for catecholamines and metanephrines was done. Out of 237 patients recruited, 169 patients had controlled in-clinic BP of which 156 patients had completed ABPM. Seventy-four were true controlled hypertensives, that is controlled by clinic automated office BP and by out-of-clinic ABPM. The remaining 82 were controlled by clinic automated office BP, but uncontrolled during out-of-clinic ABPM, indicative of MUCH. After exclusion of 4 patients because of inadequate or lack of 24-hour urinary collections, 72 true controlled hypertensive and 80 MUCH patients were analyzed. MUCH patients had significantly higher out-of-clinic BP variability and lower heart rate variability compared with true controlled hypertensives, as well as higher levels of out-of-clinic urinary catecholamines and metanephrines levels consistent with higher out-of-clinic sympathetic activity. In contrast, there was no difference in in-clinic plasma catecholamines and spot-urine/plasma levels of metanephrines between the 2 groups, consistent with similar levels of sympathetic activity while in clinic. MUCH patients have evidence of heightened out-of-clinic sympathetic activity compared with true controlled hypertensives, which may contribute to the development of MUCH.

Gene and environmental interactions according to the components of lifestyle modifications in hypertension guidelines

Risk factors for hypertension consist of lifestyle and genetic factors. Family history and twin studies have yielded heritability estimates of BP in the range of 34–67%. The most recent paper of BP GWAS has explained about 20% of the population variation of BP. An overestimation of heritability may have occurred in twin studies due to violations of shared environment assumptions, poor phenotyping practices in control cohorts, failure to account for epistasis, gene-gene and gene-environment interactions, and other non-genetic sources of phenotype modulation that are suspected to lead to underestimations of heritability in GWAS. The recommendations of hypertension guidelines in major countries consist of the following elements: weight reduction, a healthy diet, dietary sodium reduction, increasing physical activity, quitting smoking, and moderate alcohol consumption. The hypertension guidelines are mostly the same for each country or region, beyond race and culture. In this review, we summarize gene-environmental interactions associated with hypertension by describing lifestyle modifications according to the hypertension guidelines. In the era of precision medicine, clinicians who are responsible for hypertension management should consider the gene-environment interactions along with the appropriate lifestyle components toward the prevention and treatment of hypertension. We briefly reviewed the interaction of genetic and environmental factors along the constituent elements of hypertension guidelines, but a sufficient amount of evidence has not yet accumulated, and the results of genetic factors often differed in each study.

Is insulin the new intermittent hypoxia?

The sympathoexcitatory effects of insulin are well-established, although the exact mechanisms by which insulin stimulates the sympathetic nervous system are not completely understood. The majority of research supports a primary role for the central nervous system in the gradual and sustained rise in muscle sympathetic nerve activity (MSNA) in response to hyperinsulinemia; in addition, recent studies in animals suggests carotid body chemoreceptors respond to increases in systemic insulin levels. Intermittent activation of the carotid chemoreceptors, similar to that seen in patients with sleep apnea, can result in sensory long term facilitation and may contribute to the observed rise in baseline MSNA in this population. Consistent with this idea, insulin exposure results in sustained increases in MSNA that persist even when plasma insulin levels return to baseline. We propose the carotid chemoreceptors contribute to insulin-mediated sympathoexcitation and the persistent rise in MSNA in patients with sustained hyperinsulinemia. If the carotid chemoreceptors sense and respond to changes in systemic insulin levels, these organs may provide a viable target for the treatment of disorders known to exhibit sustained hyperinsulinemia and sympathoexcitation including, but not limited to, obesity, hypertension, sleep apnea, metabolic syndrome, cardiovascular disease, and diabetes.

Improving glycaemic control in a metabolically stressed patient in ICU

This article describes a clinical experience where the careful application of problem-solving skills has resulted in positive changes in glycaemic care in a critical care environment. The metabolic stress response to trauma injuries leads to episodes of hyperglycaemia. The application of a problem-solving process has resulted in greater understanding of best practice of the management of this problem. The importance of strict control of blood glucose levels in the critically ill patient is highlighted. Although the practice areas in this article is a specialized intensive care environment, in light of recent government-led recognition that many patients in hospital are increasingly ill (Department of Health (DoH), 1998a), this situation may arise in many ward environments.

Responsiveness of insulin-induced cardiac sympathetic nerve activation associates with blood pressure regulation in diabetics

To quantitatively evaluate the effect of insulin on cardiac sympathetic nerve activity (SNA) and analyze clinical factors associated with insulin sensitivity for the regulation of SNA in diabetics, 29 Japanese type 2 diabetics without neuropathy were recruited. A 2-h control study and a 2-h hyperinsulinemic euglycemic glucose clamp study were performed. From the power spectral analysis of R-R intervals on ECG during both studies, two major components, the low-frequency (LF) and the high-frequency component (HF), were obtained. Then %LF was calculated as LF/(LF +HF), and the ratio of the average %LF during the last 30 min of the clamp or the control to the average %LF for the entire time for clamp or control (R-%LF) was used as a marker of changes in SNA. R-%LF was significantly higher during the clamp than in the control (1.07 +/- 0.04 vs. 1.03 +/- 0.03, P < 0.05). High responders (individual R-%LF during clamp > or = mean + 2SD in control) showed a higher basal mean blood pressure (BP) before the clamp (89 +/- 3 vs. 82 +/- 2, P < 0.03) but not a higher glucose infusion rate (GIR) compared with low responders (<mean + 2SD). Furthermore, R-%LF showed a positive correlation with basal mean BP (P < 0.02) but not with GIR. These data demonstrate that an acute insulin load stimulates cardiac SNA, and insulin sensitivity in the regulation of SNA may be associated with BP regulation independently of peripheral insulin sensitivity.

Blood flow and muscle metabolism: a focus on insulin action

The vascular system controls the delivery of nutrients and hormones to muscle, and a number of hormones may act to regulate muscle metabolism and contractile performance by modulating blood flow to and within muscle. This review examines evidence that insulin has major hemodynamic effects to influence muscle metabolism. Whole body, isolated hindlimb perfusion studies and experiments with cell cultures suggest that the hemodynamic effects of insulin emanate from the vasculature itself and involve nitric oxide-dependent vasodilation at large and small vessels with the purpose of increasing access for insulin and nutrients to the interstitium and muscle cells. Recently developed techniques for detecting changes in microvascular flow, specifically capillary recruitment in muscle, indicate this to be a key site for early insulin action at physiological levels in rats and humans. In the absence of increases in bulk flow to muscle, insulin may act to switch flow from nonnutritive to the nutritive route. In addition, there is accumulating evidence to suggest that insulin resistance of muscle in vivo in terms of impaired glucose uptake could be partly due to impaired insulin-mediated capillary recruitment. Exercise training improves insulin-mediated capillary recruitment and glucose uptake by muscle.

Ethnic differences in insulinemia and sympathetic tone as links between obesity and blood pressure

Hyperinsulinemia and increased sympathetic nervous system (SNS) activity are thought to be pathophysiological links between obesity and hypertension. In the present study, we examined the relation among heart rate (HR), blood pressure (BP), and percent body fat (hydrodensitometry or DEXA), fasting plasma insulin concentration, and muscle sympathetic nerve activity (MSNA, microneurography) in male, normotensive whites (n=42) and Pima Indians (n=77). Pima Indians have a high prevalence of obesity and hyperinsulinemia but a relatively low prevalence of hypertension. Compared with whites, Pima Indian men had a higher percent body fat (28% versus 21%) and higher fasting insulin concentrations (210 versus 132 pmol/L) but lower MSNA (27 versus 33 bursts/min) (all P<0.001). In both ethnic groups, HR and BP were positively related to percent body fat and MSNA, and both were significant independent determinants of HR and BP in multiple regression analyses. However, MSNA was positively related to percent body fat and the fasting insulin concentration in whites (r=0.60 and r=0.47, both P<0.01) but not in Pima Indians (r=0.15 and r=0.03, NS) (P<0.01 for ethnic differences in the slope of the regression lines). These results confirm the physiological importance of the SNS in normal BP regulation but indicate that the roles of hyperinsulinemia and increased SNS activity as mediators for the relation between obesity and hypertension can differ between different ethnic groups. The lack of an increase in SNS activity with increasing adiposity and insulinemia in Pima Indians may contribute to the low prevalence of hypertension in this population.

Forearm norepinephrine spillover during standing, hyperinsulinemia, and hypoglycemia

Plasma norepinephrine (NE) concentrations are a fallible index of sympathetic neural activity because circulating NE can be derived from sympathetic nerves, the adrenal medullas, or both and because of regional differences in sympathetic neural activity. We used isotope dilution measurements of systemic and forearm NE spillover rates (SNESO and FNESO, respectively) to study the sympathochromaffin system during prolonged standing, hyperinsulinemic euglycemia, and hyperinsulinemic hypoglycemia in healthy humans. Prolonged standing led to decrements in blood pressure without increments in heart rate, the pattern of incipient vasodepressor syncope. FNESO was not increased (0.58 +/- 0.20 to 0. 50 +/- 0.21 pmol. min-1. 100 ml tissue-1), suggesting that the approximately twofold increments in plasma NE and SNESO were derived from sympathetic nerves other than those in the forearm (with a possible contribution from the adrenal medullas). Hyperinsulinemia per se (euglycemia maintained) stimulated sympathetic neural activity, as evidenced by increments in FNESO (0.57 +/- 0.11 to 1.25 +/- 0.25 pmol. min-1. 100 ml tissue-1, P < 0.05), but not adrenomedullary activity. Hypoglycemia per se stimulated adrenomedullary activity (plasma epinephrine from 190 +/- 70 to 1720 +/- 320, pmol/l, P < 0.01). Although SNESO (P < 0.05) and perhaps plasma NE (P < 0.06) were raised to a greater extent during hyperinsulinemic hypoglycemia than during hyperinsulinemic euglycemia, FNESO was not. Thus these data do not provide direct support for the concept that hypoglycemia per se also stimulates sympathetic neural activity.

Whole body and splanchnic metabolic and circulatory effects of glucose during beta-adrenergic receptor inhibition

The aim of the study was to assess the possible contribution of adrenergic mechanisms to the thermogenic and circulatory effects of glucose ingestion. With the use of indirect calorimetry and arterial, pulmonary arterial, and hepatic venous catheterization, whole body and splanchnic oxygen uptake and blood flow were examined in nine propranolol-treated healthy male volunteers before and during 2 h after oral ingestion of 75 g of glucose. The glucose effects were compared with those in nine untreated controls. After propranolol, the glucose-induced rise in splanchnic blood flow was reduced by approximately 60%, and the hepatic venous glucose release to the systemic circulation was significantly delayed. Glucose-induced increments in pulmonary and splanchnic oxygen uptake and cardiac output were similar in the two groups. It is concluded that adrenergic mechanisms contribute to the glucose-induced rise in splanchnic blood flow and thereby probably to the time course for intestinal absorption of nutrients. It is suggested that the magnitude of glucose-induced thermogenesis is independent of adrenergic stimulation.